Generally, processes for fabricating wires using the superconducting ceramics include the following steps:
(a) a step of providing starting powders, i.e., an R.sub.2 O.sub.3 powder, an alkaline earth metal carbonate powder as the component A, and CuO powder, each having an average grain size of not greater than 10 .mu.m, compounding and mixing them in a predetermined compounding ratio, to obtain a mixed powder, calcining the mixed powder in the air or in an oxygenic atmosphere, at a temperature of from 850.degree. to 950.degree. C. to form superconducting ceramics having a perovskite structure, and grinding the ceramics to obtain powder of an average grain size of not greater than 10 .mu.m,
(b) a step of filling a pipe of silver (Ag) with the superconducting powder ground in the previous step, sealing the both ends of the pipe under vacuum, and subjecting the silver pipe filled with the ground powder to drawing operations, e.g., swaging, rolling with grooved rolls, processing with a die, or the like, to produce a wire having a diameter of not greater than 5 mm as shown in FIG. 1,
(c) a final step of sintering the superconducting ceramics powder filled in the wire and then subjecting the filled Ag pipe to heat-treatment in the air or in an oxygenic atmosphere so that the ceramics can absorb oxygen enough to be required, at a temperature of from 900.degree. to 950.degree. C. to produce a final product.
Subsequent to the above-described steps (a) and (b) have been performed the following step:
(c') a step of bundling a plurality of the superconducting wires as shown in FIG. 1 and then covering the bundle with a tube made of Ag to form a cable, subjecting the cable to a processing with a die, if required, and to a heat-treatment in the air or in an oxygenic atmosphere, at a temperature of from 900.degree. to 950.degree. C. for sintering the superconducting ceramics powder to produce a superconducting cable.
In the above described conventional step (c) or (c'), the superconducting ceramic powder is heat-treated to sinter it and to enable it to absorb oxygen. In this case, because the temperature of the heat-treatment is in the range of 900.degree. to 950.degree. C., which is close to the melting point of Ag, the strength of Ag decreases, and the Ag wires filled with the superconducting powders or the superconducting cables become softened and tend to be easily bent or generate discontinuity or cut-off in the superconducting ceramics by careless bending. As a result, the conventional superconducting wires and cables are difficult to handle, and they break sometimes down during heat-treatment.
Accordingly, one might consider the possibility of using, as covering materials for the above described superconducting ceramics or wires, metals other than Ag (hereinafter, referred to as non-Ag metals), i.e., materials having excellent strength at high temperatures, including, for example, nickel alloys such as Inconel and Hastelloy, stainless steel, or the like. However, the non-Ag metals are disadvantageous because they cannot perform dispersion, penetration and discharge of oxygen. Specifically, in the superconducting wires and cables having sheath made of non-Ag metals, bulges are formed in the wires or cables because of oxygen released from the superconducting ceramics filled in the sheath. In the case of the wires, oxygen cannot be supplied to the superconducting ceramics when the superconducting ceramics filled in the wires are subjected to the final heat-treatment performed in the air or in an oxygenic atmosphere to sinter the superconducting ceramics. In the case of the cables, when temperature of the cables is decreased after the sintering of the superconducting ceramics, the ceramics cannot absorb oxygen.
Up to now, the metals other than Ag are practically unsuitable as covering materials for the superconducting ceramics or the outermost covering materials for the superconducting cables.
However, Ag has various problems in that it is very expensive, difficult to handle during high temperature heat-treatment, and has poor strength at high temperatures. In particular, the strength of the cables at room temperature is unsatisfactory.